Federal law requires emergency locator transmission (ELT) equipment on all aircraft traveling more than twenty-five miles from an airport and emergency position indicating radio beacons (EPIRBs) on certain classes of marine craft. ELTs are activated by gravitational forces (impact) while EPIRBs are activated by water contact. Both, however, may be manually activated.
ELT and EPIRB equipment transmit a distress waveform on particular emergency frequencies, e.g., 121.5 MHz and 243 MHz, to alert emergency frequency monitors that a distress incident has occurred. The distress waveform transmitted by these devices consists of an amplitude modulated carrier signal in which the modulating signal is an audio frequency sweeping downward over a range of not less than 700 Hz, within the range of 1,600 Hz to 300 Hz, and at a sweep rate varying between 2 Hz and 4 Hz. These characteristics are required by federal law, so that the transmitted distress waveform, which can be demodulated by a receiver to a siren-like sound, may easily be recognized by individuals monitoring on 121.5 MHz and 243 MHz, who can then alert search and rescue (SAR) personnel to search for the location of the source of the distress transmission and initiate rescue operations. The distress waveform, however, does not contain information other than that an ELT or an EPIRB is transmitting it. Accordingly, SAR personnel receive no advance information on whether they are searching for an airplane, marine vessel, camper, hiker, or skier. This uncertainty contributes to the inefficient use of SAR personnel and in poor coordination among rescue operations.
The United States Air Force together with the Civil Air Patrol (CAP) are responsible for SAR over land while the United States Coast Guard (USCG) handles SAR at sea. Monitoring of the emergency transmissions is done by satellite and ground stations. It may take three or four passes of a satellite to detect an emergency transmission. The three or four passes of the satellite translate to about three to four hours of delay before SAR activity can begin. In the case of distressed aircraft, the Air Force Rescue Coordination Center (AFRCC) receives notification from the satellite and then requests the CAP to launch CAP aircraft having on-board direction finder equipment. Typically, there may be one to four hours delay before the CAP aircraft launches and then an hour of flight time to get within the area of the emergency transmission. Subsequently, an airborne search begins. Once the search has been further narrowed, the SAR moves to a ground team to locate the accident site.
There may be false alarms detected due to faulty emergency transmission equipment or other non-emergency transmissions occurring on the emergency frequency bands. Depending on how quickly such false alarms can be discovered, the false alarms may result in a significant waste of already limited SAR resources.